Histamine H3 receptors and its antagonism as a novel mechanism for antipsychotic effect: a current preclinical & clinical perspective

Abstract

     Histamine H₃ receptors are present as autoreceptors on histaminergic neurons and as heteroreceptors on nonhistaminergic neurones. They control the release and synthesis of histamine and several other key neurotransmitters in the brain. H₃ antagonism may be a novel approach to develop a new class of antipsychotic medications given the gathering evidence reporting therapeutic efficacy in several central nervous system disorders. Several medications such as cariprazine, lurasidone, LY214002, bexarotene, rasagiline, raloxifene, BL-1020 and ITI-070 are being developed to treat negative symptoms and cognitive impairments of schizophrenia. These medications works through diverse mechanisms which include agonism at metabotropic mGluR2/3, partial agonism at dopamine D₂, D₃ and serotonin 5-HT_1A receptors, antagonism at D₂, 5-HT_2A, 5-HT_2B and 5-HT₇ receptors, combined dopamine antagonism with GABA agonist activity, inhibition of monoamine oxidase-B, modulation of oestrogen receptor, and activation of nuclear retinoid X receptor. However, still specific safe therapy for psychosis remains at large. Schizophrenia is a severe neuropsychiatric disorder result both from hyper- and hypo-dopaminergic transmission causing positive and negative symptoms, respectively. Pharmacological stimulation of dopamine release in the prefrontal cortex has been a viable approach in treating negative symptoms and cognitive deficits of schizophrenia symptoms that are currently not well treated and continue to represent significant unmet medical challenges. Administration of H₃ antagonists/inverse agonists increase extracellular dopamine concentrations in rat prefrontal cortex, but not in the striatum suggesting that antagonism via H₃ receptor may be a potential target for treating negative symptoms and cognitive deficits associated with schizophrenia. Further, insights are emerging into the potential role of histamine H₃ receptors as a target of antiobesity therapeutics which is one of the limiting adverse effects of second generation schizophrenia medications. The recent failures of two promising H₃ compounds in clinical trial dampened the interest in seeking antipsychotic like activities of H₃ receptor antagonists. However, due to the inconclusive nature of many of these studies, the development of H₃ compounds via H₃ antagonism/inverse agonism approach still hold lot of promises and may be developed as a novel class of drugs for schizophrenia and its related complications e.g. weight gain